The efficient collection of light transmitted in a beam and reflected back from a target, is easy to achieve if the distance to the target is known and constant. If the target distance varies, two main difficulties result.
First, one focusing lens may no longer be adequate to focus light onto the detector over the working distance range of the optical apparatus. A possible solution to this problem is to allow the user of the apparatus to estimate the target distance, to choose from a number of interchangeable lenses, and to insert the chosen lens into the apparatus. This solution allows proper focusing onto the detector but is clumsy for the user. One object of this invention is to reduce the effort required from the user without reducing the range of the apparatus.
Second, the intensity of the light focused onto the detector must normally lie within certain levels if the detector is to function correctly. With a very distant target, the intensity of reflected light is low and the detector may not respond. Conversely, if the target is extremely close, the intensity of the reflected light may be so high as to saturate the detector. Previously, this problem has been overcome by allowing the user to choose from a variety of interchangeable filters, one of which is then placed in the path of the received light. The sensitivity of the detector is selected to give the required maximum range and the filters are used to attenuate light from closer targets. A further object of this invention is to avoid the complications and inconvenience of interchangeable filters.